1. Field of the Invention
The present invention relates to a process for manufacturing a shadow mask of a Braun tube for a color TV.
2. Brief Description of the Invention
As is well known a shadow mask is an extremely thin metal strip having a great number of small holes. It is mounted in front of the fluorescent surface of a Braun tube for a color TV and performs an important part in that three electron beams emitted from three electron guns in accordance with signals of the three primary colors are allowed to pass through each hole so that fluorescent dots distributed on the fluorescent surface are caused to luminesce in three separate colors. Such a shadow mask has heretofore been manufactured as follows. A steel maker subjects a strip of low carbon steel to a cold rolling finish with a rolling reduction of at least 40% to provide a strip material of not more than 0.2 mm in thickness, which is delivered in the form of a coil to an etching processer. At the etching processer, the strip material is pretreated to remove oil while being unwound from the coil. Predetermined patterns of holes are then formed in the strip material by application of a photoresist on both sides of the strip, patternwise exposure of the photoresist, developing of the exposed photoresist, hardening of the developed photoresist by burning it at a temperature of about 200.degree. C., etching of the material through the hardened patterned photoresist by spraying an aqueous ferric chloride, and removal of the photoresist. The product is cut into individual flat masks and delivered to a Braun tube maker. At the Braun tube maker, the flat mask having a predetermined pattern of holes is annealed to impart to it a sufficient ductility for the subsequent press-forming. This annealing is normally effected at a high temperature ranging between 750.degree. to 900.degree. C. with the individual masks suspended or stacked. Since the steel strip as annealed has a yield point elongation of several percent "stretcher strains" (Luders lines) arise when it is press-formed. Furthermore, the flat mask loses its evenness owing to the annealing. In order to erase the unevenness of the annealed strip and to prevent the stretcher strains from arising in the press-forming step, the annealed flat mask is passed several times through a roller leveller, and thereafter press-formed into the desired curved plane. After the formation of oxide films on the surfaces, the shadow mask so manufactured is mounted in a Braun tube.
The prior art process stated above poses several problems, especially regarding the annealing step carried out by a Braun tube maker.
The annealing temperature as high as 750.degree. to 900.degree. C. frequently results in the adhesion of flat masks, leading to the reduction in the yield. Even with successfully annealed flat masks, waves are formed by the annealing at high temperatures and the subsequent leveller rolling to erase such waves involves a danger in that the pattern of holes may be distorted or wrinkles may arise. Furthermore, the high temperature anneal causes the carbon in the low carbon steel material to diffuse and precipitate near the surfaces of the strip, and this precipitation of carbon is not necessarily uniform. Any non-uniformity of the carbon precipitation results in non-uniform elongation of the material in the press-forming step, and thus, faulty products are frequently found after the press-forming step.
To overcome the problems discussed above, attempts have been made to use lower annealing temperatures. However, when a sufficiently low annealing temperature for avoiding the adhesion and thermal distortion of flat masks is used, the grains become finer, resulting in an increase in the yield point elongation of the annealed material, and it is necessary to increase the number of passes through a roller leveller for preventing the stretcher strains from taking place in the subsequent press-forming step. A solution of this problem is proposed in Japanese Patent Examined Publication No. 51-13102, published on Apr. 24, 1976. The proposed solution is based on the use of a steel of an extremely low carbon content of 0.002 to 0.012% by weight. It is taught in this Japanese Patent Publication that if such an extremely low carbon steel is used in the manufacture of a shadow mask the usable annealing temperature can be reduced to the extent of 600.degree. to 750.degree. C. without suffering from the above-mentioned problem, although such low annealing temperatures are unsuitable for a C 0.05% steel normally employed in the manufacture of shadow masks. Incidentally this Publication is completely silent with respect to the rolling reduction in the finish cold rolling step.